1. Field of the Invention
The invention relates to closed loop control circuits or the like and refers more specifically to a closed loop control circuit and structure for and the steps of operating on a feedback signal from a controlled mechanical frequency response structure to provide an error signal, and for providing an output signal when the error signal is not between a lower and an upper signal limit.
2. Description of the Prior Art
In closed loop control circuits it is often desirable to monitor the error signal, i.e., the difference between the command and feedback signals, in order to assure that the output, which is equivalent to the feedback signal, is properly corresponding to the command signal. If this error exceeds a predetermined permissible error, then a detection circuit is actuated which provides appropriate indication or action such as stopping the system.
For many applications where cyclic input is used, such as fatigue test systems, it is desired to maintain output magnitude equal to input magnitude, but phase errors are permissible. In the past, error detection circuits have been subject to error introduced by the phase shift of the controlled structure in addition to any magnitude errors. In many instances, the phase error far exceeds the desired permissible band of magnitude error, causing the conventional direct comparison circuit to be of little value for monitoring system magnitude error.